Prescription eyeglass lenses are curved in such a way that light is correctly focused onto the retina of a patient's eye, improving vision. Such lenses are formed from glass or plastic lens “blanks” having certain desired properties to provide the correct prescription for the patient. The blanks are usually circular and of substantially larger dimension compared to the relatively smaller finished lenses assembled into eyeglass frames. Therefore, a lens blank must be edged to fit an eyeglass frame selected by the patient.
Ophthalmic laboratory technicians cut, grind, edge, and polish blanks according to prescriptions provided by dispensing opticians, optometrists, or ophthalmologists. The specifications include the patient's full prescription, including: 1) the total power the finished lens must have; 2) the strength and size of any segments, if needed (i.e. multifocal lenses); 3) the power and orientation of any cylinder curves; and 4) the location of the optical center and any inducted prism that may be needed.
In addition, the large diameter blank is sized and shaped to fit into the frame selected by the patient. The lens blank may be shaped using an edger, such as the edger disclosed in U.S. Pat. No. 6,203,409 to Kennedy et al., the disclosure of which is incorporated herein by reference. The blank is edged so that the periphery of the finished lenses fit into the openings on the frames.
Edging of a lens blank typically requires the application of a block to a surface thereof. The block is releasably secured to a clamp assembly, so that rotation of the clamp assembly causes corresponding rotation of the lens blank. As the blank is rotated, the periphery of the blank may be cut to a desired size using a router tool. The blank may be either ground or cut. Wet edgers use diamond-impregnated wheels with different abrasive grits to grind the lens material. A coolant is sprayed on the wheels during edging to reduce heat. Dry edgers use carbide steel or diamond blades mounted on the spindle of a motor to shave the lens. The lens periphery may also be polished using a polishing tool. Some edgers are also able to form a bevel about the periphery of the lens.
Information relating to the size and shape of the lens needed for a particular frame (i.e. trace data) may be generated, and subsequently transmitted to the edger. Such trace data may be provided by frame manufacturers, or generated by a tracer machine. Trace data may be downloaded and/or transmitted to a storage medium in a control system, such as a central processing unit, in communication with the edger.
The edger processes the edge of the lens blank to create an edge profile according to the trace data. The finished lens may then be assembled with the selected eyeglass frames. Many frames have a groove extending around the inner circumference of the openings. The groove interlocks with a complementarily shaped bevel formed about the peripheral edge of the lens. The interlock between the complementary groove and bevel helps to secure the lens within the frame opening.
Many of today's frames have a relatively ‘high wrap’, such as frames having a curvature greater than 6 diopters. Typically, though not necessarily, high wrap is provided so that the eyeglass frame more closely follows the contour of the wearer's face. It is often difficult to insert a beveled lens into the corresponding groove in a high wrap frame because the edge of the lens adjacent the formed bevel interferes with the portion of the frame adjacent the groove. The lens bevel may not properly rest within the frame groove, particularly when inserting relatively thick lenses into high wrap frames. The result is a poor fit between the lenses and frames.